Allows /configprops endpoint to show nested properties

Including maps and lists. Beans with no metadata (in
/META-INF/*spring-configuration-metadata.json) are just serialized
as they come (so might have problems like cycles). Serialization
errors are caught and rendered as an "error" for that bean. Any
problems can be fixed by preparing metadata and specifying which
properties are to be rendered that way.

Fixes gh-1746, fixes gh-1921